The present invention relates to a plasma CVD process used for fabrication of semiconductor devices and to a semiconductor device fabricated by the process, and particularly to a plasma CVD process capable of forming a metal film containing a residual halogen element in a small amount and to a high reliability semiconductor device fabricated by the process.
With the tendency toward increasing density in the design rule of semiconductor devices such as LSIs from a half-micron level to a quarter-micron level or less and also toward adoption of a multi-layered interconnection structure, the aspect ratio of a contact hole for connecting interconnection layers to each other needs to be increased. For example, for a semiconductor device having a 0.18 .mu.m rule, the diameter of a contact hole is 0.2 .mu.m and the thickness of an interlayer insulating film is about 1.0 .mu.m, Consequently, the aspect ratio of the contact hole is 5. A process for achieving a multi-layered interconnection structure with a high reliability through such a fine contact hole having such a high aspect ratio has been developed. A a Ti metal layer for ohmic contact and a TiN layer as a barrier metal for preventing diffusion of an interconnection material are thinly formed on the inner surface of the contact hole in a conformal manner. Then the contact hole is filled with an upper interconnection material or a contact plug by high temperature sputtering of an Al based metal material, or selective CVD or blanket CVD of tungsten.
In general, a Ti metal layer or a TiN layer is formed by sputtering or reactive sputtering using a bulk Ti metal as a target. In particular, there has been proposed, in Japanese Patent Laid-open No. Hei 6-140359, a collimated sputtering process in which vertical incident components of sputtering particles are made larger or a long-distance sputtering process in which a target distance is increased It has been reported that these sputtering processes are effective to reduce the contact resistance and improve the barrier performance as compared with the ordinary sputtering process. However, these sputtering processes having a feature in which vertical incident components of sputtering particles toward a substrate to be processed are made larger, have a problem that the thickness of a shoulder portion or a side surface of a fine contact hole having a large aspect ratio inevitably becomes extremely thin. When such a contact hole is subjected to blanket CVD of W or the like in the subsequent step, there arises an inconvenience in that a source gas WF.sub.6 permeates through the thin portion of the film, leading to abnormal growth of W or separation of the Ti metal layer or the TiN layer.
To cope with the above problem regarding step coverage which has been not solved by these sputtering processes including the collimated sputtering, there has been proposed a method of forming a Ti metal layer and a TiN layer in a conformal manner by a CVD process making use of a chemical reaction on the surface of a substrate to be processed.
The CVD processes for forming a Ti based material layer by making use of a chemical reaction, that have been proposed up to now are generally classified into two groups: one being a type of process using an inorganic metal halide such as TiCl.sub.4, which has been reported for example in "Abstract of the 44-th Symposium of Semiconductor/Integrated Circuit Technology Institute, pp. 31 (1993)"; and the other being a type of process using an organic metal compound such as TDMAT or TDEAT, which has been reported for example in "Proc. 11th. Int. IEEE VMIC, pp. 440 (1994)".
In the latter type using an organic metal source gas, generally, a Ti metal layer is formed by sputtering and then a TiN layer is formed by MOCVD.
On the other hand, the former type using an inorganic source gas is advantageous in that a Ti metal layer is formed by a reduction reaction by H.sub.2 in a CVD system and a TiN layer is continuously formed in the same CVD system using the same gas added with N.sub.2.
Incidentally, a reduction reaction of TiCl.sub.4 as a metal halide using hydrogen molecules is an endothermic reaction expressed by ##EQU1## where .DELTA.G indicates a standard heat of formation.
This reduction reaction is thermodynamically slow.
A method of accelerating this reduction reaction of TiCl.sub.4 has been proposed, in which hydrogen atoms and hydrogen radicals produced by dissociation of hydrogen in a plasma are used in a reduction reaction for formation of a Ti layer by plasma CVD. Such a reaction is an exothermic reaction expressed by ##EQU2##
This reduction reaction of TiCl.sub.4, which is higher at reaction speed, makes it possible to form a Ti metal layer by plasma CVD at a relatively low temperature. In addition, a plasma CVD process using a high density plasma source, for example, an ECR-CVD process, inductively coupled plasma CVD process or a helicon wave plasma CVD process is advantageous in film formation rate and film uniformity.
However, the above plasma CVD process for forming a Ti metal layer using a reduction reaction of TiCl.sub.4 by hydrogen atoms and hydrogen radicals has the following two problems.
The first problem is that in some film formation conditions, TiCi.sub.4 is not sufficiently reduced by hydrogen atoms and hydrogen radicals even by plasma CVD using a high density plasma. Consequently, the Ti metal layer is not uniformly formed. In this case, a underlying conductive material, for example, a silicon substrate, is etched in accordance with the following equation: ##EQU3##
This problem will be described in detail with reference to FIG. 6.
FIG. 6 is a schematic sectional view illustrating a problem occurring in the case where a Ti metal layer is formed by plasma CVD in a contact hole 3 formed in an interlayer insulating film 2 on a Si substrate 1. In this case, when TiCl.sub.4 is not sufficiently reduced by hydrogen atoms and hydrogen radicals, the Si substrate 1 is etched in accordance with the equation (3), to produced a corrosion portion 6. The production of such a corrosion portion 6 will break a shallow diffusion layer (not shown) in a surface portion of the Si substrate 1, leading to a serious defect of increasing leak current. In addition, a Ti metal layer is formed but is not shown in FIG. 6.
Moreover, when the reduction of TiCl.sub.4 by hydrogen atoms and hydrogen radicals is imperfect, an intermediate product TiClx (x is an integer less than 4) or chlorine atoms are entrapped in the Ti metal layer during formation thereof, thereby increasing the residual amount of chlorine in the Ti metal layer. As a result, an Al based layer formed in the subsequent step is possibly corroded, leading to the increased contact resistance, and to disconnection in the worst case.
The second problem is that in some film formation conditions, a Ti metal film is not uniformly grown, that is, particles of Ti metal are non-uniformly grown even by plasma CVD using a high density plasma source.
This problem will be described in detail with reference to FIGS. 13A and 13B.
FIGS. 13A and 13B are schematic sectional views illustrating a problem occurring in the case where a Ti metal film 4 is formed by plasma CVD in a contact hole 3 formed in an interlayer insulating film 2 on a semiconducting substrate 1 made from Si or the like. When TiCl.sub.4 is not sufficiently reduced by hydrogen atoms and hydrogen radicals, the surface morphology of the Ti metal film 4 is degraded due to non-uniform growth of particles of Ti metal, as shown in FIG. 13A.
Thus, in the case where a TiN film 6 is formed on the Ti metal film 4 having such a poor surface morphology, since the TiN film 6 is grown while following the surface shape of the underlying Ti metal film 4 as shown in FIG. 13B, it is formed in the shape of particles and is prevented from being grown at portions where adjacent particles of TiN are brought in contact with each other. In particular, a large gap 7 is possibly produced at a corner of a bottom portion of the contact hole 3.
When a layer of a high melting point metal such as W is formed on the bottom portion of the contact hole 3 having such a gap 7 by CVD using a source gas WF.sub.6, the source gas corrodes the underlying semiconducting substrate 1 by way of the gap 7, to break a shallow diffusion layer (not shown) in a surface portion of the substrate 1. Also, in the case where the contact hole 3 is embedded with an Al based metal, the Al based metal passes through the gap 7, and reacts with the semiconducting substrate 1 made of Si or the like, to thereby cause an alloy spike or the like. In each case, there arises serious defects such as the increased leak current in connection between layers.
Moreover, when the reduction of TiCl.sub.4 by hydrogen atoms and hydrogen radicals is imperfect, a precursor TiClx (x is an integer less than 4) or chlorine atoms are entrapped in the Ti metal layer during formation thereof, to increase the residual amount of chlorine in the Ti metal layer. As a result, an Al based layer formed in the subsequent step is possibly corroded, leading to the increased contact resistance, and to disconnection in the worst case.